Nanostructures produced by phase-separation during growth of (iii-v )1-x(iv2)x alloys

ABSTRACT

Nanostructures ( 18 ) and methods for production thereof by phase separation during metal organic vapor-phase epitaxy (MOVPE). An embodiment of one of the methods may comprise providing a growth surface in a reaction chamber and introducing a first mixture of precursor materials into the reaction chamber to form a buffer layer ( 12 ) thereon. A second mixture of precursor materials may be provided into the reaction chamber to form an active region ( 14 ) on the buffer layer ( 12 ), wherein the nanostructure ( 18 ) is embedded in a matrix ( 16 ) in the active region ( 14 ). Additional steps are also disclosed for preparing the nanostructure ( 18 ) product for various applications.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention under ContractNo. DEAC36-99G010337 between the United States Department of Energy andthe National Renewable Energy Laboratory, operated for the U.S.Department of Energy by Midwest Research Institute, Battelle, Bechtel.

TECHNICAL FIELD

This invention relates to nanostructures and more specifically tonanostructures and methods of production by phase-separation duringmetal organic vapor-phase epitaxy (MOVPE).

BACKGROUND ART

Nanostructures (i.e., material having sizes that are on the order of afew nanometers) exhibit properties that are intermediate between theproperties exhibited by atoms and molecules, and the propertiesexhibited by bulk solids. These unique properties, or quantum mechanicaleffects, make nanostructures promising candidates for various optical,optoelectronic, and microelectronic applications.

Nanostructure-based devices are expected to offer improved performanceover more conventional devices. For example, indirect band gapsemiconductors (e.g., silicon (Si) and germanium (Ge)) typically exhibitextremely low radiative efficiency in bulk materials. However, quantumconfinement of the carriers in nanostructures may increase the radiativeefficiency and emission energies of these semiconductors. Direct bandgap semiconductors (e.g., indium-arsenide (InAs) and indium-phosphide(InP)) are already commonly used in lasers because of their highradiative efficiencies in bulk materials. However, quantum confinementof the carriers in nanostructures may lower threshold current densitiesand allow for temperature independent energy emission, while alsoincreasing emission energies. These properties may also be fine-tuned bycontrolling the size of the nanostructures. In addition, with themovement toward smaller, high-bandwidth, low-power interconnects,nanostructures are providing a basis for various optoelectronicapplications. For example, nanostructures may be used in opticalinterconnects for integrated circuits, telecommunications equipment,electronic equipment, etc. Nanostructures may also be used forbiological sensors (e.g., capable of connecting with molecules in thehuman body), and as field emission electron sources (e.g., for flatpanel displays), among other applications.

However, before any of these applications can be effectively realized,nanostructures of known size and having a narrow size distribution mustbe reliably produced. Current methods of fabricating nanostructures areunreliable, producing nanostructures having inconsistent or undesirableproperties, and/or are expensive. For example, the Stranski-Krastanovmethod may be used to produce a coherent strained layer ofthree-dimensional nanostructure islands. However, the nanostructures areunstable and have varying optical properties. Nanofabrication usinglithography and etching to form Si and Ge nanostructures is expensive,and the resulting nanostructures have poor optical properties. Othermethods such as laser-assisted catalytic growth of “freestanding”nanowires, and colloidal chemical synthesis both produce nanostructureswhich are not embedded in a semiconductormaterial, making thesenanostructures less desirable for device applications. Yet other methodsfor producing nanostructures are also known, such as anodizing andetching to form porous Si containing Si quantum wires, and ionimplantation and annealing to form Si or Ge nanocrystals, for example,embedded in a SiO₂ matrix.

A need remains for a relatively inexpensive and reproducible method ofproducing high-quality nanostructures. Additional advantages would berealized if the process were spontaneous, thereby reducing or altogethereliminating manual intervention. Still other advantages would berealized if the method allowed greater control over the growth process,and hence the properties of the resulting nanostructures. Other optical,optoelectronic, and microelectronic applications would also be possibleif the nanostructures could be produced from a wide range of materials.

DISCLOSURE OF INVENTION

An embodiment of a method for producing a nanostructure by phaseseparation during metal organic vapor-phase epitaxy (MOVPE) may comprisethe steps of providing a growth surface in a reaction chamber andintroducing a first mixture of precursor materials into the reactionchamber to form a buffer layer thereon, providing a second mixture ofprecursor materials into the reaction chamber to form an active regionon the buffer layer, wherein the nanostructure is embedded in a matrixin the active region, and reintroducing the first mixture of precursormaterials or a third mixture into the reaction chamber to form a caplayer over the active region.

Another embodiment of a method for producing a nanostructure by phaseseparation during MOVPE may comprise the steps of providing a growthsurface, forming a buffer layer on the growth surface, growing an activeregion having the nanostructure embedded in a matrix on the bufferlayer, and removing a portion of the active region.

Nanostructures (e.g., nanocrystals and nanowires) produced according tothe embodiments of the invention are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative and presently preferred embodiments of the invention areshown in the accompanying drawings in which:

FIG. 1(a) through FIG. 1(c) illustrate an embodiment of nanostructureproduction by phase separation during metal organic vapor-phase epitaxy(MOVPE), wherein nanocrystals are produced;

FIG. 2(a) through FIG. 2(c) illustrate another embodiment ofnanostructure production by phase separation during MOVPE, whereinnanowires are produced;

FIG. 3(a) through FIG. 3(c) illustrate yet another embodiment ofnanostructure production by phase separation during MOVPE, wherein atleast a portion of the matrix is removed;

FIG. 4(a) through FIG. 4(d) illustrate still another embodiment ofnanostructure production by phase separation during MOVPE, wherein atemplate for fabricating nanostructures is produced;

FIG. 5 is a 002 dark field (002 DF) transmission electron microscopy(TEM) image showing a cross-section of nanostructures produced accordingto an embodiment of the invention;

FIG. 6(a) and FIG. 6(b) are 200 DF TEM images showing plan-views ofnanostructures produced according to an embodiment of the invention,wherein the alloy composition was changed to control the density of thenanostructures;

FIG. 7 is a high-resolution electron microscopy (HREM) TEM image showinga plan-view of a Ge nanostructure produced according to an embodiment ofthe invention;

FIG. 8(a) through FIG. 8(c) are 200 DF TEM images showing plan-views ofnanostructures produced according to an embodiment of the invention,wherein the temperature was changed to control the size of thenanostructures;

FIG. 9 is a 002 DF TEM image showing a cross-section of nanocrystalsproduced according to an embodiment of the invention;

FIG. 10 shows high resolution scanning electron microscopy (SEM) imagesof nanostructures produced according to embodiments of the invention,wherein (a) is an array of nano-sized holes formed by selective chemicaletching to remove the embedded nanostructures and may be used as atemplate for nanostructure fabrication, and (b) is an array ofprotruding nanostructures formed by selective chemical etching to removethe matrix.

BEST MODES FOR CARRYING OUT THE INVENTION

Nanostructures and production thereof is shown and described hereinaccording to preferred embodiments of the invention. Briefly,nanostructures may comprise material configured as ultra-fine strands(i.e., “nanowires”) or ultra-fine crystals (i.e., “nanocrystals”) thatare much smaller than the microstructures that are currently produced byconventional microstructure fabrication techniques. Because of theirsize (i.e., on the order of nanometers), nanostructures exhibit uniqueproperties that make them useful in various optical, optoelectronic, andmicroelectronic applications, such as in light emitters and detectors,integrated circuits, and flat panel displays to name only a few.Therefore, it is desirable to produce nanostructures of known size andhaving a narrow size distribution for use in such applications.

According to embodiments of the invention, nanostructures 18, 118 (e.g.,FIG. 1(c) and FIG. 2(c), respectively) may be produced by phaseseparation during metal organic vapor-phase epitaxy (MOVPE). In one suchembodiment, a growth surface may be mounted in a reaction chamber, and afirst mixture of precursor materials is introduced into the reactionchamber. The components of the first mixture of precursor materialsdeposit on the growth surface and form a buffer layer 12 thereon. Oncethe buffer layer 12 has been grown to the desired thickness,introduction of the first mixture of precursor materials may bediscontinued and a second mixture of precursor materials may beintroduced into the reaction chamber. The components of the secondmixture of precursor materials deposit on the buffer layer 12 and forman active region 14, 114 thereon. As the components of the secondmixture of precursor materials deposit on the buffer layer 12, theyphase-separate from one another and form distinct nanostructures 18, 118embedded in a matrix 16, 116 in the active region 14, 114. The activeregion 14 may be grown for only a short duration, resulting in theformation of “nanocrystals” 18 (i.e., nanostructures less than 20 nmlong). Alternatively, the active region 114 may be grown thicker,resulting in the formation of “nanowires” 118 (i.e., nanostructureslonger than 20 nm and even as long as one or more microns).

Embodiments of methods for producing nanostructures 18, 118 by phaseseparation during metal organic vapor-phase epitaxy (MOVPE) may compriseadditional steps. For example, after the active region 14, 114 has beengrown to the desired thickness, the first mixture of precursor materialsmay be reintroduced into the reaction chamber to form a cap layer 20over the active region 14, 114 so that the nanostructure product may beused in various devices or applications (e.g., in semiconductor lasers).In another embodiment, at least a portion of the matrix 116 may beremoved from the active region 114 by a selective etching process toexpose at least a portion 119 of the nanostructure 118, (FIG. 3(c)) sothat the nanostructure product may be used in various other types ofdevices or applications (e.g., as an electron emitter). In yet otherembodiments, at least a portion of the nanostructure 118 may be removedfrom the active region 114 by another selective etching process toproduce a template 22 that can in turn be used to fabricate othernanostructures 218 (FIG. 4(c) and FIG. 4(d)).

A significant advantage of the nanostructures produced according toembodiments of the invention is their high-quality and relativeuniformity in size. The process for producing these nanostructures iscompatible with known epitaxial growth techniques, and therefore isreadily reproducible. In addition, the nanostructures are“self-assembled” during the process and require little, if any, manualintervention. Additional advantages of the invention include the degreeof control over the properties of the resulting nanostructures, and thewide range of precursor materials that can be used to producenanostructures having a wide range of optical, optoelectronic, andmicroelectronic properties. Yet other advantages will also becomeapparent with an understanding of the invention.

Having briefly described nanostructures and methods for productionthereof, as well as some of the more significant advantages associatedtherewith, the various embodiments of the present invention will now bedescribed in greater detail below.

Nanostructures may be produced according to an embodiment of theinvention by phase separation during metal organic vapor-phase epitaxy(MOVPE). MOVPE is a well-understood and widely used process for“growing” a thin crystalline layer on a substrate material. Generally,the substrate material is provided in a furnace, and various precursormaterials are introduced in gaseous form (e.g., using a carrier gas).The components of the precursor materials come into contact with anddeposit on the heated substrate material, resulting in the growth of acrystalline layer on the substrate material.

More specifically, MOVPE may be used according to the teachings of thepresent invention to grow nanostructures on the substrate material asfollows. The substrate material (not shown), or growth surface as it isalso referred to, may be any suitable material. For example, wheregallium arsenide (GaAs) or germanium (Ge) nanostructures are grown on alattice-matched surface, a gallium arsenide (GaAs) or germanium (Ge)growth surface may be used. Alternatively, where silicon (Si)nanostructures are grown on a lattice-matched surface, a galliumphosphide (GaP) or silicon (Si) growth surface may be used.

In any event, the growth surface may be mounted within the reactionchamber of a furnace (not shown). For example, where a radio frequency(RF) heating element is used, the growth surface may be mounted using agraphite susceptor in the furnace. Alternatively, where infrared (IR)heating is used, the growth surface may be mounted on a metal block inthe furnace. The growth surface must be clean and free from defects,since any defects may be reproduced and magnified in the subsequentlayers that are grown thereon during the MOVPE process. Therefore, thereaction chamber is typically evacuated to a total pressure of about 50Torr and the growth surface is cleaned by heating it (e.g., for about 2minutes at about 700° C.) under flowing hydrogen and an AsH₃over-pressure of about 0.5 Torr before cooling down to the growthtemperature.

Following preparation of the growth surface, a first mixture ofprecursor materials is introduced to the reaction chamber using acarrier gas. As the first mixture of precursor materials passes throughthe reaction chamber, it is thermally decomposed and deposited on theheated growth surface. These deposits accumulate or “grow” to form athin coating on the growth surface that is referred to as a buffer layer12 (e.g., FIG. 1(a)). When the buffer layer 12 is grown to the desiredthickness, the introduction of the first mixture of precursor materialsis suspended to stop the growth of the buffer layer 12. In its place, asecond mixture of precursor materials is introduced to the reactionchamber using a carrier gas. Again, as the second mixture of precursormaterials passes through the reaction chamber, it is thermallydecomposed and deposited on the buffer layer 12, and grows to form thenext layer. This next layer, or the active region 14, 114, is where thenanostructures 18, 118 of the present invention are formed.

According to embodiments of the invention, the nanostructures are formedby phase-separation of the deposited material during growth of theactive region 14. That is, the components of the second mixture ofprecursor materials separate from one another as the second material isdeposited in the active region 14 and form distinct nanostructures 18,118 embedded in a matrix 16, 116. For example, where the components ofthe second mixture of precursor materials comprise germanium (Ge),gallium (Ga), indium (In), and phosphorus (P), the Ge phase-separatesfrom the GaInP to form Ge nanostructures in a GaInP matrix.

The phase-separation mechanism can be understood as follows with respectto the above example. Although GaInP and Ge are size matched, they aremutually insoluble in the equilibrium bulk solid state, leading toalmost complete phase-separation into GahIP and Ge-rich regions at alltemperatures below the melting point. The cause of the phase separationis related to the high energy required to form Ga—Ge, In—Ge, and P—Gebonds, which do not satisfy the octet rule for valence electronsobserved in the pure components, and the even higher energies predictedfor In—In, P—P, and Ga—Ga anti-site bonds.

Accordingly, as the layer starts growing, the GaInP-rich phase depositsfirst, with excess Ge segregating to the growing layer surface becausethe formation of the higher-energy bonds is unfavorable. After thesurface Ge concentration reaches a critical value, nucleation of Ge-richislands occurs on the GaInP growth surface. The excess surface Ge thenprecipitates out at the Ge-rich islands, because it can now formlow-energy Ge—Ge bonds at the Ge-rich nuclei. The GaInP-rich phasebetween the Ge-rich islands continues to grow and the Ge atoms arrivingat the growth surface diffuse to the Ge-rich surface islands and areincorporated there. Repetition of the above growth behavior results inthe observed formation of nanostructures during growth of the activeregion 14.

Once the active region 14, 114 has been grown to the desired thickness,introduction of the second mixture of precursor materials is suspended.For example, where it is desired to produce “nanocrystals,” growth ofthe active region 14 is discontinued after a short duration so that thegrowth is limited to only nanocrystal-like structures 18 (FIG. 1(b)).Alternatively, where it is desired to produce “nanowires,” the growth ofthe active region 114 continues until a strand-like material 118 (FIG.2(b)) forms to the desired length.

Nanocrystals 18 may be produced having substantially equal lengths inthree dimensions. For example, growth of the active region 14 (i.e., thelength of the nanostructure) may be limited to about 20 nm or less.Alternatively, nanowires 118 may be produced having a relatively highlength to diameter ratio (i.e., substantially equal in two dimensionsand longer in the third dimension). For example, growth of the activeregion 114 (i.e., the length of the nanostructure) may continue beyond20 nm and may even be grown as thick as one micron (μm) or more. It isunderstood, however, that the size of the nanostructures may vary,according to the teachings of the invention, based on designconsiderations, such as, but not limited to, the intended application ordevice in which the nanostructures will be used. For example,nanocrystals and/or nanowires may be fabricated for use in the activeregion of devices, such as light emitting diodes (LEDs), semiconductorlasers, light detectors, transistors, and biological detectors, to namea few.

Production of nanocrystals 18 using phase-separation during MOVPE isillustrated according to one embodiment in FIG. 1(a) through FIG. 1(c).Generally, the buffer layer 12 is grown, as described above and shown inFIG. 1(a). Once the buffer layer 12 is grown to the desired thickness,the active region 14 is grown on the buffer layer 12, again as describedabove and shown in FIG. 1(b). During growth of the active region 14,phase-separation of the components of the second mixture of precursormaterials causes distinct nanostructures 18 to form and become embeddedin a matrix 16. Once the active region 14 has been grown to the desiredthickness (e.g., the nanocrystals 18 are the desired length), a caplayer 20 may optionally be grown over the active region 14, as shown inFIG. 1(c).

The cap layer 20, or the confining or cladding layer as it is alsoreferred to, may be provided as a protective coating over the activeregion 14. For example, the cap layer 20 may protect the nanostructures18 from becoming contaminated with phosphorus as the furnace is cooledafter the growth process. In addition, some devices or applications mayrequire confinement of the carriers in the active region (i.e., thenanostructures) therebetween. For example, semiconductor lasers mayrequire the carriers to be “sandwiched” between high band gap layers. Orfor example, light emitters may require a coating material having adifferent refractive index for optical confinement. In any event, thecap layer 20 may be grown using the same method (and material wheredesired) as was used to grow the buffer layer 12. That is, after growthof the active region 14 is discontinued, the first mixture of precursormaterials is again introduced to the reaction chamber (or a thirdmixture of precursor materials is used). As the first mixture ofprecursor materials passes through the reaction chamber, depositsaccumulate over the active region 14 and form the cap layer 20 thereon.

Production of nanowires 118 is illustrated according to one embodimentof the invention in FIG. 2(a) through FIG. 2(c). The steps are similarto those described above with respect to the production of thenanocrystals 18 and shown in FIG. 1(a) through FIG. 1(c). Morespecifically, the buffer layer 12 is grown to the desired thickness, asshown in FIG. 2(a). An active region 114 is then grown on the bufferlayer 12, as shown in FIG. 2(b). Again, the components of the secondmixture of precursor materials phase-separate from one another duringgrowth of the active region 114 to form distinct nanostructures 118embedded in a matrix 116. In this embodiment, however, the active region114 continues to grow thicker than the active region 14 where thenanocrystals 18 are produced, resulting in the production of strand-likestructures or nanowires 118. Once the active region 114 has been grownto the desired thickness (e.g., the nanowires 118 are the desired size),a cap layer 20 may optionally be grown over the active region 114, asshown in FIG. 2(c).

As an illustration of the production of nanostructures according to theteachings of the invention, germanium (Ge) nanostructures 18, 118 may beproduced in a gallium indium phosphide (GaInP) matrix 16, 116 on agallium arsenide (GaAs) buffer layer 12 as follows. After the growthsurface is prepared (e.g., by heating it in the reaction chamber asdiscussed above), the first mixture of precursor materials is introducedto the reaction chamber using a suitable carrier gas. For example, thefirst mixture of precursor materials may comprise a gallium source suchas triethyl- or trimethyl-gallium (TEG or TMG), and an arsenic sourcesuch as arsine (AsH₃), and it may be introduced to the reaction chamberusing hydrogen gas as the carrier gas. As the mixture of precursormaterials passes through the reaction chamber over the heated substrate,gallium arsenide (GaAs) is deposited on the growth surface and forms aGaAs buffer layer 12. When the buffer layer 12 is grown to the desiredthickness, introduction of the first mixture of precursor materials isdiscontinued, and the second mixture of precursor materials isintroduced into the reaction chamber to form the active region 14, 114.

As an example, the second mixture of precursor materials may comprise agallium source such as TMG, an indium source such as triethyl- ortrimethyl-indium (TEI or TMI), a phosphorus source such as phosphine(PH₃), and a germanium source such as germane or di-germane. As thesecond mixture of precursor materials passes through the reactionchamber, germanium (Ge) and gallium indium phosphide (GaInP) aredeposited on the buffer layer 12, forming the active region 14, 114thereon. The Ge and GaInP phase-separate from one another on the bufferlayer 12, and form discrete Ge nanostructures 18, 118 embedded in aGaInP matrix 16, 116 in the active region 14, 114. Once the activeregion 14, 114 is grown to the desired thickness (e.g., to formnanocrystals 18 or nanowires 118), the introduction of the secondmixture of precursor materials is discontinued.

Where it is desired to produce a cap layer 20 over the active region 14,114, the first mixture of precursor materials may again be introduced tothe reaction chamber. As the mixture of precursor materials passesthrough the reaction chamber, gallium arsenide (GaAs), in this example,is deposited on the growth surface and forms a GaAs cap layer 20.Alternatively, a third mixture of precursor materials may be introducedwhere the cap layer 20 is desired to be made of a different material. Inany event, when the cap layer 20 is grown to the desired thickness,introduction of the third mixture of precursor materials isdiscontinued. The reaction chamber may be cooled, and the nanostructureproduct removed from the reaction chamber.

It is understood that any precursor materials suitable for use withMOVPE may be used according to the teachings of the invention, and othersuitable precursor materials will become apparent to one skilled in theart after having become familiar with the teachings of the invention andmay depend at least to some extent on various design considerations.Indeed, where it is desirable to use other materials for the differentlayers, it is readily apparent that other precursor materials will needto be used. For example, triethyl- or trimethyl-aluminum (TEA or TMA)may be substituted for TMG where an aluminum arsenic buffer layer isdesired. The invention therefore contemplates an active region 14, 114characterized by the generic formula (group III-V compound)_(1-x)((groupIV element or alloy)₂)_(x), wherein the group III-V compound comprises agroup III element and a group V element or an alloy comprised of amixture of several different group III and group V elements.

It should also be noted that other embodiments for producingnanostructures are also contemplated as being within the scope of theinvention. For example, when the nanostructures are to be used fordevice applications (e.g., in solid state electronics), p/n junctionsmay be required. Briefly, when p-type and n-type materials are placed incontact with one another, current flows readily in one direction (i.e.,from one material to the other) but not in the opposite direction (e.g.,creating a basic diode). Where it is desired to grow n-type and/orp-type layers, the precursor material is doped (e.g., with Zinc (Zn) toform a p-type layer or with Silicon (Si) to form an n-type layer).Accordingly, these p/n junctions can be formed by growing the activeregion 14, 114 on an n-type buffer layer 12, and then growing a p-typecap layer 20 over the active region 14, 114. Alternatively, the activeregion 14, 114 may be grown on a p-type buffer layer 12, and then ann-type cap layer 20 may be grown over the active region 14, 114.

As the size of the nanostructures is controlled by kinetic factors, thesize of the nanostructures produced according to the teachings of theinvention may be controlled by adjusting various growth parameters, suchas the reaction temperature, the growth rate (i.e., the rate at whichthe precursor material is supplied to the growth chamber), the V/IIIratio, etc., or a combination thereof. For example, increasing thegrowth temperature causes the nanostructures to be formed with a largerdiameter. Likewise, decreasing the growth temperature causes thenanostructures to be formed with a smaller diameter. In addition,varying the concentration of the precursor materials results innanostructures of various sizes and densities. The V/Il ratio usedduring growth may also affect the density and other properties of thenanostructures. The reactor pressure, carrier gas, and type of sourcematerials used during growth may also influence the properties of thenanostructures. Introducing strain between the nanostructure materialand the matrix material, and/or between the active layer and substratematerial may also be used to affect the size, shape, density,arrangement, electronic and other properties of the nanostructures. Thetype of substrate material and substrate surface orientation (i.e.,growth direction) used for growth may also be used to influence thenanostructure properties such as size, shape, density, arrangement, etc.The addition of one or more surfactants, (e.g., Sb, Bi, etc.) duringgrowth may also be used to affect the properties of the nanostructuressuch as size, density, etc. Post growth annealing may also be used tocontrol the nanostructure properties.

By controlling the various characteristics of the nanostructures (e.g.,size, density, etc.), the properties of the nanostructures may bechanged or altered for use in a variety of different applications ordevices. For example, by controlling the size of the nanostructures, theband gap of Ge nanostructures may be tuned from 0.7 eV to over 4 eV.This enables light-emitting devices to be fabricated on lattice-matchedGaAs and Ge substrates, and may provide an alternative to GaN-basedmaterials for which no suitable lattice-matched substrate exists.

It is also noted that the nanostructure product produced according tothe teachings of the invention may be lattice-matched orlattice-mismatched. That is, lattice-matched structures are those inwhich the constituents (e.g., GaAs and Ge) have the same latticeparameter, even if the crystal structure itself is different for each.For example, the components in (GaAs)_(1-x)(Ge₂)_(x),(Ga_(0.52)In_(0.48)P)_(1-x)(Ge₂)_(x), and (GaP)_(1-x)(Si₂)_(x), areessentially lattice-matched. Alternatively, lattice-mismatchedstructures are those in which the constituents have different latticeparameters. For example, the components in (InAs)_(1-x)(Si₂)_(x),(GaAs)_(1-x)(Si₂)_(x), and (Ga_(1-y)In_(y)P)_(1-x)(Si₂)_(x) areessentially lattice-mismatched. Both lattice-matched andlattice-mismatched products exhibit unique characteristics that makeeach desirable in various applications.

Precursor material having lattice-matched components result in theproduction of lattice-matched nanostructures. That is, there is nostrain between the nanostructures and the surrounding matrix.Accordingly, these nanostructures tend to be more thermally stable thanthe strained quantum particles produced by the conventionalStranski-Krastanov process. In addition, as the nanostructures in theselattice-matched systems are unstrained with respect to the matrix andalso possess a different crystal structure, it is relatively easy tomeasure their size and shape by electron microscopy. Accordingly, thisenhances the ability to determine and control their size, and makes thenanostructures ideal for testing theoretical models of the optical andelectronic properties of semiconductor nanostructures.

Even more lattice-mismatched components exist, which may enable a muchwider range of optical and electronic properties to be realized fromnanostructures formed in the phase-separated material. For example,nanostructures formed of lattice-mismatched components, e.g.,(InAs)_(1-x)(Si₂)_(x), may be used for direct band gap group III-Vnanostructures on Si substrates spanning the energy gap range from about0.4 electron volts (eV) to over 4 eV. This includes the 1.3 to 1.55 μmand visible light wavelength regions. Such Si-based light-emittingdevices are of particular importance as optical interconnects onSi-based integrated circuits. Similarly growth of phase-separated(InAs)_(1-x)(Ge₂)_(x) may enable the fabrication of direct band gapgroup III-V nanostructure devices on GaAs and Ge substrates spanning the1.3 to 1.55 μm wavelength region, which is particularly important foroptical fiber communication.

Another embodiment of the invention is illustrated with respect to FIG.3(a) through FIG. 3(c) in which nanostructures are produced for use aselectron emitters. The initial steps shown in FIG. 3(a) and FIG. 3(b),wherein the buffer layer 12 and the active region 114 are grown, aresimilar to those described above with respect to the production ofnanowires 118 (FIG. 2(a) and FIG. 2(b)). That is, the buffer layer 12 isgrown to the desired thickness, as shown in FIG. 3(a), followed by theactive region 114, as shown in FIG. 3(b). Again, phase-separation of thecomponents of the precursor material during growth of the active region114 causes the formation of distinct nanostructures 118 embedded in amatrix 116. According to this embodiment, however, once the activeregion 114 has been grown to the desired thickness (e.g., thenanostructures 118 are the desired size), the growth process is stopped.For example, introduction of the second mixture of precursor materialsmay be discontinued, the reaction chamber is allowed to cool, and thenanostructure product removed.

According to this embodiment, instead of forming a cap layer 20 on theactive region 114, the matrix 116 is removed to expose at least aportion of the nanostructures 118 (e.g., exposed wire 119 in FIG. 3(c)).The matrix 116 may be removed according to any suitable process, such asetching. Etching is a well-known process that uses an etchant (e.g.,concentrated hydrochloric acid (HCl)) to selectively target and erode aparticular element or compound. According to the invention, the etchantmay be introduced to the active area 114, which targets the matrix 116and erodes at least a portion thereof. However, the etchant does noterode the nanostructures, and thus leaves at least a portion of thenanowires 118 in the active region 114 exposed. Although in theembodiment shown in FIG. 3(c), the matrix 116 is not fully eroded andthus serves to maintain the spacing and alignment of the nanostructures118, in other embodiments, the matrix 116 may be fully removed to“harvest” individual nanostructures 118. As an example, for the(Ga_(0.52)In_(0.48))_(1-x)(Ge₂)_(x) system concentrated HCl or HCl/H₂Omixtures may be used to selectively target the Ga_(0.52)In_(0.48)Pmatrix.

Once produced, the array of exposed nanostructures attached to thebuffer layer 12 may be used as field emission electron sources. Forexample, the nanostructures 118 may be placed in a vacuum, and anegative and positive electrode each positioned on opposite sidesthereof, wherein electrons are caused to flow from the negative side,through the nanostructures 118 and across the vacuum toward the positiveelectrode.

Yet another embodiment of the invention is illustrated with respect toFIG. 4(a) through FIG. 4(d) in which a template is produced for makingnanostructures. The initial steps shown in FIG. 4(a) and FIG. 4(b),wherein the buffer layer 12 and the active region 114 are grown, againare similar to those described above with respect to the production ofnanowires 118 (FIG. 2(a) and FIG. 2(b)). That is, the buffer layer 12 isgrown to the desired thickness, as shown in FIG. 4(a), followed by theactive region 114, as shown in FIG. 4(b). Again, phase-separation of thecomponents of the precursor material during growth of the active region114 causes the formation of distinct nanostructures 118 embedded in amatrix 116. According to this embodiment, however, once the activeregion 114 has been grown to the desired thickness (e.g., the nanowires118 are the desired size), the growth process is stopped. For example,introduction of the second mixture of precursor materials may bediscontinued, the reaction chamber allowed to cool, and thenanostructure product removed.

Instead of forming a cap layer 20 on the active region 114, at least aportion of the nanostructures 118 are removed according to thisembodiment, to form a hole or void 24 in the matrix 116 as shown in FIG.4(c). The nanostructures 118 may be removed according to any suitableprocess, such as the etching process described above. However, in thisembodiment, an etchant (e.g., H₂SO₄/H₂O₂/H₂O) may be introduced to theactive area 114, which targets only the nanostructures 118 and erodes atleast a portion thereof. The etchant does not erode the matrix 116, andthus forms the voids 24 in the active region 114. For example, for the(Ga_(0.52)In_(0.48)P)_(1-x)(Ge₂)_(x) system H₂SO₄/H₂O/H₂O,NH₄OH/H₂O₂/H₂O, and H₃PO₄/H₂O₂/H₂O mixtures may be used to selectivelytarget the Ge nanostructures.

Once produced, the voids 24 formed in the matrix 116 may be used as atemplate for producing nanostructures 218. That is, the template 22 maybe mounted in the reaction chamber of the furnace, and a third mixtureof precursor materials having only the components for forming thedesired nanostructures 218 may be introduced using a suitable carriergas. As the precursor material passes over the template 22, thenanostructure component is deposited in the voids 24 of the template 22to grow the desired nanostructure 218 therein.

As an example of its use, some of the matrix material may beincorporated with the nanostructures that are formed initially duringphase-separation. Thus, the “impure” nanostructures 118 may be removedto form template 22, as just explained, and the voids 24 formed in thetemplate 22 may be refilled to fabricate highly-pure nanostructures 218.Optionally in such an embodiment, a portion of the originalnanostructure 118 may be left in the voids 24 formed in the template 22and act as a “seed” to facilitate growth of the pure material therein.The template 22 may also be used where the phase-separation mechanismmay not work, or may not work as well, for the components of a precursormaterial. For example, when the nanostructure product is to be used as alight emitter, the template 22 may be used to fabricate nanostructures218 from a direct band gap material such as InP, InAs, or GaAs. Thetemplate 22 may also be used to fabricate hetero-nanostructures 218(e.g., Si—Ge nanowires) by alternately filling the template with Si andge.

It should be noted that although embodiments of the invention are shownin FIG. 3(a) through FIG. 3(c) and in FIG. 4(a) through FIG. 4(d) forselectively removing either the matrix 116 or the nanostructures 118,wherein the nanostructures are nanowires 118, the invention may also bepracticed according to these embodiments wherein the nanostructures arenanocrystals 18. In addition, the buffer layer 12, matrix 16, 116, andnanostructures 18, 118 may be formed using any suitable materialaccording to the teachings of the present invention. It is furthercontemplated that other embodiments for selectively removing either aportion of the matrix or the nanostructures are also contemplated asbeing within the scope of the invention, and will occur to those skilledin the art after having become familiar with the teachings of theinvention. Consequently, the scope of the invention should not belimited to the selective etching process described herein.

Before continuing with specific examples of the invention, it should benoted that the invention is not limited to the embodiments described andillustrated herein. Other embodiments of the invention are alsocontemplated as being within the scope of the invention, as will becomeapparent to one skilled in the art after having become familiar with theteachings of the invention. Likewise, the following examples areprovided to illustrate various embodiments of the invention. However,the invention is not to be limited thereto.

EXAMPLES

Nanostructures were produced in the following examples by phaseseparation during metal organic vapor-phase epitaxy (MOVPE) according tothe teachings of the invention. A GaAs crystalline substrate was mountedon a metal block in the reaction chamber of an infrared (IR) heatedMOVPE furnace and prepared by evacuating the reaction chamber to apressure of 50 Torr and heating it for 2 minutes at 700° C. underflowing hydrogen and with an AsH₃ over-pressure of about 0.5 Torr.

After the growth surface was prepared, the temperature of the substratewas held at about 643° C., and the pressure of the reactor at about 50Torr, for the following growth process. A first mixture of precursormaterials was introduced to the reaction chamber, using hydrogen gas asthe carrier gas, to grow a gallium arsenide (GaAs) buffer layer. Thegrowth rate was about 0.04 μm/min and V/III ratio about 50, and thefirst precursor material comprised trimethyl-gallium (TMG) as thegallium source, and arsine (AsH₃) as the arsenic source. As theprecursor material passed over the heated substrate, the gallium andarsenic source materials were thermally decomposed and gallium (Ga) andarsenic (As) were deposited on the GaAs growth surface to form a GaAsbuffer layer. When the buffer layer was grown to about 500 nm,introduction of the TMG and arsine was suspended. Accordingly a 500 nmthick buffer layer was grown on the growth surface and provided afoundation for growing the active region.

A second mixture of precursor materials was then introduced to thereaction chamber to grow the active region. The second precursormaterial comprised trimethyl-gallium (TMG) as the gallium (Ga) source,trimethyl-indium (IBM as the indium (In) source, phosphine (PH₃) as thephosphorus (P) source, and germane as the germanium (Ge) source.Hydrogen was again used as the carrier gas and the V/III ratio was againclose to 50. As the second precursor material passed through thereaction chamber, Ga, In, P, and Ge were deposited, at a rate of about0.08 μm/min, on the GaAs buffer layer and formed the active region.During growth of the active region, phase-separation occurred. That is,the Ge separated from the Ga_(0.52)In_(0.48)P to form Ge nanostructuresembedded in a matrix of Ga_(0.52)In_(0.48)P. It is believed that thephase-separation mechanism took place as follows. As the layer startedgrowing, the GaInP-rich phase deposited first, with excess Gesegregating to the growing layer surface. After the surface Geconcentration reached a critical value, nucleation of Ge-rich islandsoccurred on the GaInP growth surface. The excess surface Ge thenprecipitated out at the Ge-rich islands by forming low-energy Ge—Gebonds at the Ge-rich nuclei. Both the GaInP-rich matrix and the Ge-richnanostructures continued to grow, the latter by surface diffusion of Geatoms from the GaInP-rich matrix, and repetition of the above growthbehavior is believed to have resulted in the observed formation of theGe nanostructures embedded in the GaInP matrix.

In one example, the active region was grown to about 20 nm thick toproduce nanocrystals. In another example, the active region was grown toabout 1 μm thick to produce nanowires. Once the active region was grownto the desired thickness, introduction of the second precursor materialwas discontinued. In examples where a cap layer was desired, the firstprecursor material comprising TMG and arsine was again introduced to thereaction chamber using hydrogen as the carrier gas. In any event,following growth of the nanostructure product, a flow of arsine andhydrogen gas was introduced into the reaction chamber while it wascooled to room temperature. The nanostructure product so produced was acomposite semiconductor with Ge nanowires embedded in aGa_(0.52)In_(0.48)P matrix.

In another example a different second mixture of precursor materials wasintroduced to the reaction chamber to grow the active region aftergrowth of the GaAs buffer layer. The second mixture of precursormaterials comprised trimethyl-gallium (TMG) as the gallium (Ga) source,arsine (AsH₃) as the arsenic (As) source, and germane as the germanium(Ge) source. Hydrogen was again used as the carrier gas and the reactorpressure was again 50 Torr. A V/III ratio of about 3 was used. As thesecond mixture of precursor materials passed through the reactionchamber, Ga, As, and Ge were deposited on the GaAs buffer layer at about0.09 μm/min and formed the active region. During growth of the activeregion, phase-separation occurred. That is, the Ge separated from theGaAs to form Ge nanostructures embedded in a matrix of GaAs by themechanism described above.

In yet another example another different second mixture of precursormaterials was introduced to the reaction chamber to grow the activeregion after growth of the GaAs buffer layer. The second mixture ofprecursor materials comprised trimethyl-gallium (TMG) as the gallium(Ga) source, trimethyl-aluminum (TMAl) as the aluminum (Al) source,phosphine (PH₃) as the phosphorus (P) source, and germane as thegermanium (Ge) source. Hydrogen was again used as the carrier gas andthe reactor pressure was again 50 Torr. A growth temperature of 619° C.and V/III ratio of about 150 was used. As the second mixture ofprecursor materials passed through the reaction chamber, Al, In, P, andGe were deposited, at a rate of about 0.09 μm/min, on the GaAs bufferlayer and formed the active region. During growth of the active region,phase-separation occurred. That is, the Ge separated from theAl_(0.48)In_(0.52)P to form Ge nanostructures embedded in a matrix ofAl_(0.48)In_(0.52)P by the mechanism described above.

The nanostructure product was imaged using transmission electronmicroscopy (TEM) The TEM samples were prepared by conventionalmechanical and ion-milling techniques and examined using a Philips CM30transmission electron microscope. TEM images of nanostructures producedby the phase-separation technique are shown in FIG. 5 through FIG. 9.The cross-sectional TEM image of FIG. 5 shows coherent Ge-rich nanowiresthreading vertically in the [001] growth direction embedded in the GaInPmatrix of a phase-separated (Ga_(0.52)In_(0.48)P)_(0.8)(Ge₂)_(0.2)alloy, grown at 643° C. using a V/III ratio of about 50 and at a rate of0.08 μm/min, on a GaAs buffer layer. FIG. 6(a) shows a plan-view TEMimage of the same sample. The Ge nanowires are roughly rectangular incross-section and in this sample have diameters ranging from about 20 to50 nm. The edges of the nanowires are aligned roughly parallel to theorthogonal <110> directions. The nanostructures shown in FIG. 5 and FIG.6(a) have a density of about 9×10⁹ cm². The nanostructures shown in FIG.6(b) were formed by phase-separation in a(Ga_(0.52)In_(0.48)P)_(0.9)(Ge₂)_(0.1) alloy grown under the sameconditions as the sample of FIGS. 5 and 6(a) and have a density of about4.5×10⁹ cm⁻², i.e., half that of the sample in FIGS. 5 and 6(a), but thesame average diameter. This clearly illustrates that the density of thenanostructures can be controlled by varying the alloy composition.Chemical analysis performed on such samples in a JEOL 2010F highresolution scanning transmission electron microscope using energydispersive x-ray nano-analysis revealed negligible amounts of Ge in theGaInP matrix material and negligible amounts of Ga, In, and P in the Genanostructures respectively.

Since the Ge and Ga_(0.52)In_(0.48)P components are lattice matched, nostrain is present between the Ge nanowires and the Ga_(0.52)In_(0.48)Pmatrix, and hence no defects are present at the nanowire/matrixinterfaces that therefore remain coherent as can clearly be seen in theplan-view high resolution TEM lattice image of FIG. 7.

Other examples of nanostructure product produced according to theteachings of the invention are shown in FIG. 8(a) through FIG. 8(c), inwhich all growth parameters for the (Ga_(0.52)In_(0.48)P)_(1-x)(Ge₂)_(x)were held constant (V/I ratio ⁻50, growth rate ⁻0.08 μm/min, reactorpressure 50 Torr) except for the growth temperature, which was changedto vary the diameter of the nanostructures. That is, lower growthtemperatures (e.g., 604° C.) resulted in nanostructures that had anaverage diameter of less than 10 nm, as best seen in FG. 8(a). Highergrowth temperatures (e.g., 643° C.) resulted in nanostructures that hadan average diameter of about 25 nm, as best seen in FIG. 8(b). Yethigher growth temperatures (e.g., 681° C.) resulted in nanostructuresthat had an average diameter of about 55 nm, as best seen in FIG. 8(c).

As shown in FIG. 9, a (Ga_(0.52)In_(0.48)P)_(1-x)(Ge₂)_(x) thin layergrown using similar conditions to the previous examples, thenanocrystals appear similar to the nanowires when imaged using TBM,except that the Ge-rich nanostructures are shorter in length than thenanowires shown in FIG. 5. That is, the nanocrystals are preferably lessthan about 20 nm, whereas the nanowires are preferably longer than about20 nm, and may even be 1 μm or longer.

In the following examples selective chemical etching experiments wereperformed on a 1 μm thick (Ga_(0.52)In_(0.48)P)_(0.8)(Ge₂)_(0.2)nanostructure product layer grown at −604° C. using a V/III ratio of −50and a growth rate of −0.08 μm/min. In the first example, the(Ga_(0.52)In_(0.48)P)_(0.8)(Ge₂)_(0.2) layer was etched for about 2minutes in a 1/1/1 volume ratio mixture of concentrated H₂SO₄/H₂O₂/H₂Oto selectively etch away the Ge nanowires leaving theGa_(0.52)In_(0.48)P matrix behind. The result of this procedure is shownin the high-resolution scanning electron microscope image of FIG. 10(a)that shows a close spaced array of nanosized holes (dark contrast areas)in the Ga_(0.52)In_(0.48)P matrix resulting from the selective removalof the Ge nanowires by the chemical etching. Such an array of nanosizedholes may be used as a template for the fabrication of othernanostructures as discussed earlier, e.g., by filling the holes withanother material. In the second example, the(Ga_(0.52)In_(0.48)P)_(0.8)(Ge₂)_(0.2) layer was etched for about 10seconds in concentrated HCl to selectively etch away theGa_(0.52)In_(0.48)P matrix and expose the Ge nanowires. The result ofthis procedure is shown in FIG. 10(b), a high resolution scanningelectron microscope image showing the close spaced array of Ge nanowires(bright contrast regions) protruding from the surface of the remainingGa_(0.52)In_(0.48)P matrix. Such an array of protruding nanowires may beused, for example, in field emission electron sources for flat paneldisplays as discussed earlier.

It is readily apparent that the nanostructures produced according toembodiments of the method of the invention exhibit unique propertiesthat make them particularly useful for a number of applications. Theprocess offers a significant degree of control over the properties ofthe resulting nanostructures. In addition, the process is applicable toa wide range of (III-V)_(1-x) (IV₂)_(x) alloys, including thepotentially very important (GaP)_(1-x)(Si₂) and (InAs)_(1-x)(Si₂)_(x)systems. Accordingly, the process of producing nanostructures accordingto the present invention is particularly advantageous for variousoptical, optoelectronic, and microelectronic applications. Consequently,the claimed invention represents an important development innanostructures and the production thereof.

Having herein set forth preferred embodiments of the present invention,it is anticipated that suitable modifications can be made thereto whichwill nonetheless remain within the scope of the present invention.Therefore, it is intended that the appended claims be construed toinclude alternative embodiments of the invention except insofar aslimited by the prior art.

1. A method of producing nanostructures by phase separation during metalorganic vapor-phase epitaxy (MOVPE), comprising: providing a growthsurface in a reaction chamber; introducing a first mixture of precursormaterials into the reaction chamber to form a buffer layer on the growthsurface; introducing a second mixture of precursor materials into thereaction chamber to form an active region on the buffer layer, whereinthe nanostructures are embedded in a matrix in the active region; andintroducing a third mixture of precursor materials into the reactionchamber to form a cap layer over the active region.
 2. The method ofclaim 1, wherein introducing the third mixture of precursor materialsreintroduces the first mixture of precursor materials.
 3. The method ofclaim 1, further comprising doping the first and third mixtures ofprecursor materials to form a p-type buffer layer and an n-type caplayer.
 4. The method of claim 1, further comprising doping the first andthird mixtures of precursor materials to form an n-type buffer layer anda p-type cap layer.
 5. The method of claim 1, further comprisingcontrolling at least one property of the nanostructures by adjusting atleast one of the following parameters: temperature of the growthsurface, growth surface material, growth rate of the active region,ratio of the second precursor materials, composition of the secondmixture of precursor materials, substrate surface orientation, strainbetween the active region and the growth surface, and pressure of thereaction chamber,
 6. The method of claim 1, further comprisingintroducing at least one surfactant to control at least one property ofthe nanostructures.
 7. The method of claim 1, further comprisingannealing the active region after formation of the nanostructures. 8.The method of claim 1, wherein the first, second, and third mixtures ofprecursor materials are introduced using at least one carrier gas. 9.The method of claim 8, wherein the flow rate of the carrier gas isadjusted to control at least one property of the nanostructures.
 10. Themethod of claim 8, wherein the type of carrier gas is changed to controlat least one property of the nanostructures.
 11. A nanostructure productproduced according to the method of claim
 1. 12. The nanostructureproduct of claim 11, further comprising an active region characterizedby the generic formula (group III-V)_(1-x)((group IV₂)_(x).
 13. Thenanostructure product of claim 12, wherein group III-V is selected fromcombinations of: group III elements consisting of B, Al, Ga, In, and Tl;and group V elements consisting of N, P, As, Sb, and Bi.
 14. Thenanostructure product of claim 12, wherein group IV is selected from thegroup consisting of C, Si, Ge, Sn, and Pb.
 15. The nanostructure productof claim 11, further comprising a group IV nanostructure and a groupIII-V matrix.
 16. The nanostructure product of claim 11, furthercomprising a group III-v nanostructure and a group IV matrix.
 17. Thenanostructure product of claim 11 characterized as lattice-matched. 18.The nanostructure product of claim 11 characterized aslattice-mismatched.
 19. A nanowire produced according to the method ofclaim
 1. 20. The nanowire of claim 19, consisting essentially of atleast one group IV element.
 21. The nanowire of claim 19, consistingessentially of a group III-v compound.
 22. A nanocrystal producedaccording to the method of claim
 1. 23. The nanocrystal of claim 22,consisting essentially of at least one group IV element.
 24. Thenanocrystal of claim 22, consisting essentially of a group III-vcompound.
 25. The nanocrystal of claim 22, characterized as having alength of less than about 20 nm.
 26. A method of producing ananostructure by phase separation during metal organic vapor-phaseepitaxy (MOVPE), comprising: providing a growth surface; forming abuffer layer on the growth surface; growing an active region having thenanostructure embedded in a matrix on the buffer layer; and removing aportion of the active region.
 27. The method of claim 26, whereingrowing the active region comprises providing a mixture of precursormaterials in a reaction chamber until the nanostructure grows to adesired size on the buffer layer.
 28. The method of claim 26, furthercomprising phase-separating a group III-V compound from at least onegroup IV element during growth of the active region.
 29. The method ofclaim 28, wherein the at least one group IV element forms thenanostructure and the group III-v compound forms the matrix in theactive region.
 30. The method of claim 28, wherein the group III-vcompound forms the nanostructure and the at least one group IV elementforms the matrix in the active region.
 31. The method of claim 26,wherein removing a portion of the active region is by selective etching.32. The method of claim 26, wherein removing a portion of the activeregion comprises removing at least a portion of the matrix.
 33. Ananostructure product produced according to the method of claim
 32. 34.The method of claim 26, wherein removing a portion of the active regioncomprises removing at least a portion of the nanostructure.
 35. Atemplate for fabricating nanostructures produced according to the methodof claim 34.